Push only diverged stack branches

[brdv]

Summary

Make stck push force-push only branches that actually diverged from origin, while still applying planned retargets.

Changelist

• src/cli.rs
  • During push-state creation, filter candidate push branches using gitops::branch_needs_push.
  • Keep retarget behavior unchanged and still ordered after pushes.
• tests/cli_skeleton.rs
  • Extended test stub to support multiple diverged branches via STCK_TEST_NEEDS_PUSH_BRANCHES.
  • Updated existing push tests to set divergence where pushes are expected.
  • Added regression test push_skips_branches_without_local_changes.

Validation

• cargo fmt --all
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-features